The present invention relates to a method and an apparatus for polishing balls for use in a ball bearing or the like, as well as to a method of forming an annular groove for guiding a ball when it is abraded.
As illustrated in FIG. 1, in this type of conventional ball polishing apparatus, a plurality of annular grooves (ball grooves) 4 having a size substantially equal to the diametrical size of a ball 3 to be polished are concentrically formed in a rotary plate 1 which rotates and in a fixed plate 2 which is stationary and opposite to the rotary plate 1. A rotary conveyor 5 which rotates conveys and introduces the balls 3 to be polished to the annular grooves 4 where they are polished so as to comply with predetermined standards.
In actually polishing the balls 3, some ball polishing apparatuses contain tens of thousands of balls 3 which are stored in the conveyor 5 at one time, and they are repeatedly polished by through feed. These "tens of thousands of balls 3" stored in the conveyor 5 at one time will hereinafter be referred to as one lot. After the balls 3 of one lot have undergone all the processing steps, the next lot will be processed.
As illustrated in FIGS. 2A and 2B, the polishing process required for one lot usually includes several stages (e.g. three stages; i.e., a roughing stage, a semi-finishing stage, and a finishing stage). Machining pressures are controlled so as to ensure the accuracy of a machining speed and a diametrical size corresponding to each stage. FIG. 2A illustrates the relationship between machining pressures and the corresponding machining stages; and FIG. 2B illustrates the relationship between the amount of variations in the diametrical size of the ball and the respective machining stages.
As illustrated in FIG. 2A, the largest machining pressure is preset for the roughing stage, and a middle degree of machining pressure is preset for the semi-finishing stage. Then, the least machining pressure is preset for the finishing stage. In this way, the machining pressure is changed according to the machining stage, thereby increasing the amount of polishing of the ball 3 in the roughing stage and bringing the ball 3 close to a desired ball in terms of the accuracy of the surface and finished size (the diametrical size of the ball) in the finishing stage.
FIG. 2B illustrates the amounts of scheduled polishing allowance for the ball 3 in the respective machining stages, indicating the difference between the purposes of machining.
FIG. 3 is a longitudinal cross section illustrating the configuration of the conventional ball polishing apparatus. In the drawing, supports 7a, 7b are provided on a bed 6. A rotary plate 1 is supported by the support 7a (on the left-hand side of the drawing) so as to be rotatable and movable in the longitudinal direction of the bed 6. In other words, the support 7a doubles as a guide member. The rotary plate 1 is fixed to a flange 9 which is integrally formed at one end of a rotary shaft 8. The rotary shaft 8 is rotatively inserted into the center hole formed in a piston rod 10 which is fitted into the center hole of the support 7a in a a slidable manner. The rotary shaft 8 is supported at both ends by the piston rod 10 via rolling bearings 11a and 11b, such as ball bearings or taper-roller bearings, so as to be rotatable and to be slidable together with the piston rod 10. A pulley 12 is fitted to the other end of the rotary shaft 8 via a spline so as to be slidable. The pulley 12 is connected to the drive shaft of a motor by way of an endless belt (not shown). The rotary plate 1 is rotated together with the rotary shaft 8 by means of a drive force of the motor.
Two oil chambers 13a and 13b are formed between the internal circumferential surface of the center hole of the support 7a and the external circumferential surface of the piston rod 10, and liquid-operated (hydraulic) ports 14a and 14b are bored in the support 7a so as to communicate with the respective oil chambers 13a and 13b. These hydraulic ports 14a and 14b are connected to a hydraulic circuit (not shown). The rotary plate 1 slides over the bed 6 in its longitudinal direction in the drawing together with the piston rod 10 by alternate influx or efflux of a working (hydraulic) fluid into or out of the respective hydraulic chambers 13a and 13b. The rotary plate 1 is pressed against the surface of the fixed plate 2 mounted on the support 7b by feeding the hydraulic fluid into the hydraulic chamber 13a, and by discharging the hydraulic fluid out of the hydraulic chamber 13b. The pressing force is regulated by a pressure regulation mechanism provided in the hydraulic circuit. In FIG. 3, a bellows cover 15 prevents exposure of a portion of the piston rod 10 in the vicinity of one end of the support 7a.
With the balls 3 to be polished being sandwiches between the rotary plate 1 and the fixed plate 2 (that is between the annular grooves 4), the rotary plate 1 is rotated while it is pressed against the fixed plate 2. As a result, the balls 3 repeatedly pass along the annular grooves 4, whereby the balls 3 are polished so as to achieve desired size and quality. This polishing process is usually carried out while machining pressures (the machining pressures of the rotary disk 1) and the rotational speed of the rotary disk 1, or the like, are regulated. Further, the polishing process is comprised of two or three steps; i.e., roughing and finishing steps or of three steps; i.e., roughing, semi-finishing, and finishing steps. In this cased it is desirable to control the machining load imposed on the ball 3 (a load imposed on the rotary plate 1) in the final finishing process to ensure as small a force as possible with as high accuracy as possible.
In a case where annular grooves are formed in both plates of the conventional ball polishing apparatus, annular grooves are previously formed in the fixed plate by a lathe or the like, and this fixed plate is attached to a fixed plate mount on the main body of the polishing apparatus.
Subsequently, a so-called "plate conditioning" is carried out; namely, balls to be polished are introduced into the space between a plane rotary plate without annular grooves which has a grindstone fitted and the fixed plate having the annular grooves formed therein, and then the polishing of the balls is repeated, so that annular grooves are formed in the rotary plate. The "plate conditioning operation" is continued until the annular grooves of the rotary disk are formed to a predetermined depth, and uniform contact is ensured between the balls and the annular grooves formed in both plates.
The previously described conventional ball polishing apparatus presents the following problems:
A sliding guide mechanism is of high frictional resistance, and a rolling guide is usually subjected to an increase in frictional force due to a pre-load or resistance in it's sealing section. The frictional force or resistance is not negligible as compared to a pressure required to polish the ball 3. For this reason, as illustrated in FIG. 4, hysteresis develops in the regulated machining pressure during the course of polishing of the balls 3 when the machining pressure is regulated according to the machining process. Further, since the frictional force changes even during stable machining operations, it is difficult to control the machining pressure with a high degree of accuracy.
In the conventional ball polishing apparatus illustrated in FIG. 3 which uses the sliding guide, if the balls 3 are polished under the polishing pressures in the respective three machining steps; namely, the roughing step, the semi-finishing step, and the finishing step, as illustrated in FIGS. 2A and 2B, actual machining pressures in the respective machining steps change to become higher or lower due to the previously-described hysteresis with reference to preset values, thereby rendering the practical machining pressures unstable.
In the foregoing process of polishing the balls 3 while they are sandwiches between the annular grooves 4 concentrically formed in both plates 1 and 2, it is necessary to concentrically rotate the annular grooves 4 formed in the fixed plate 2 and the annular grooves formed in the rotary plate 1 with a high degree of accuracy. However, if there is a relative rotational error in the rotary plate 1 or a relative eccentricity, a relative positional displacement arises in the annular grooves 4 that are formed in the rotary plate 1 and in the fixed plate 2 so as to be opposite to each other as illustrated in FIG. 5, thereby adversely affecting the accuracy of the machining of the balls 3. More specifically, variations arise in the diameter and sphericity of the balls 3 in one lot.
The cited conventional ball polishing apparatus employs a combination of the sliding guide and rolling movement or a combination of the rolling guide and the rolling movement, and therefore the previous problems arise at one time.
In some cases, conventional desired specifications may present no problems even if the balls are used as a ball bearing for use in; e.g., a conventional hard disk drive. However, these balls have become insufficient to cope with a recent tendency toward hard disk drives (HDD) with increased capacity. The reason for this is an increase in the degree of rigorousness of the requirements for asynchronous oscillating components (NRRO) caused by the ball bearing.
In terms of improvements in the accuracy of the balls 3 to be machined, there is a limit to the control of the machining pressure of the conventional ball polishing apparatus. The profiles of the annular grooves 4 which are formed in the rotary plate 1 and the fixed plate 2 so as to be opposite to each other must be correct, and it is necessary to minimize the relative positional errors (see FIG. 5) in the annular grooves 4 while the balls 3 introduced between the plates 1 and 2 travel to the exit from the entrance.
If a relative positional error arises in the annular grooves 4 that are opposite to each other, an uncontrollable load acts on the balls 3. Exertion of such a load on the balls 3 in the final step of the polishing process intended to increase the accuracy of the balls 3 results in the deterioration of the accuracy of quality of the balls 3. Both annular grooves 4 are formed by repeatedly polishing the balls 3 spuriously between a fixed plate in which annular grooves are concentrically formed previously by turning and a rotary plate without annular grooves. Accordingly, in principle, the relative positional error in the annular grooves 4 formed in the plates 1 and 2 is corrected.
However, an original rotational error exists in a support bearing of the rotary plate 1. In general, the rotational error in the support bearing is about 1 to 10 micrometers for a rolling bearing and is about 0.1 to 0.2 micrometers for a hydrostatic bearing. Therefore, there is no real chance of complete agreement in relative position between the annular grooves 4. So long as attention is given solely to the fixed plate, the balls 3 which pass through the annular grooves 4 are repeatedly brought into the states illustrated in FIGS. 6A and 6B. In this case, the profile of the annular grooves 4 formed in either the rotary plate 1 or the fixed plate 2 will become damaged inappropriately, or improvements in the sphericity of the balls 3 will be prevented.
In the conventional ball polishing apparatus, a machining pressure application means (a hydraulic cylinder) used in the roughing process is the same as a machining pressure application means used in the finishing process. The machining pressure application means manufactured in accordance with a machining pressure used in the roughing process produces large frictional resistance when sliding under a low machining pressure in the finishing process. Therefore, it is difficult to control the machining pressure with a high degree of accuracy.
In contrast, in the light of alignment between the two plates 1 and 2, variations in the machining pressure during the machining operation will result in changes in the deformation of the support of the rotary plate 1 or the fixed plate 2 even under a low load during the final finishing process. Eventually, there arise variations in the alignment between the plates 1 and 2, which makes it impossible to ensure the quality of the balls with a high degree of accuracy.
Further, according to a conventional pressurizing method in which a machining pressure is applied by means of a hydraulic cylinder or a spring, the thus-regulated machining pressure is held substantially constant regardless of the dimensional differences among the balls 3 within a lot which are being machined between the plates 1 and 2. Therefore, it is difficult to correct the amount of variation in the size of the balls 3 among groups of balls 3 having fine dimensional variations in one lot of the balls 3 to be machined between the plates 1 and 2.
Moreover, in the method of forming annular grooves in both plates of the conventional ball polishing apparatus, attention is paid to prevention of an eccentricity between the center of pitch circle of each annular grooves previously formed in a fixed plate and the rotary center of a rotary shaft. However, there usually arises an eccentricity of about 10 to 20 micrometers. For this reason, it is usually necessary to perform the previously-described "plate conditioning operation" for two to three months until the eccentricity is corrected. If the fixed plate is made of a casting, and the rotary plate is made of a grindstone, the amount of abrasion to the rotary plate incurred during a ball polishing step is small, in turn extending the time required for the "plate conditioning operation" in order to correct the eccentricity.